Solid-state lighting (SSL) refers to a type of lighting utilizing light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs) or polymer light-emitting diodes (PLEDs) as sources of illumination rather than electrical filaments, plasma (e.g., used in arc lamps such as fluorescent lamps) or gas. The term “solid-state” refers to the fact light in an LED is emitted from a solid object, a block of semiconductor rather than from a vacuum or gas tube, as is the case in traditional incandescent light bulbs and fluorescent lamps. Compared to incandescent lighting, however, SSL creates visible light with reduced heat generation or parasitic energy dissipation, similar to fluorescent lighting. In addition, its solid-state nature provides for greater resistance to shock, vibration and wear, thereby increasing its lifespan significantly. Solid-state lighting is often used in area lighting, signage, traffic lights and is also used frequently in modern vehicle lights, train marker lights, etc.
The adoption of high power solid state lighting (SSL) or high power LED lighting to global applications, including indoor and outdoor tasks and area lighting, is limited by high system costs relative to the lower costs of less efficient, traditional lighting sources. It is therefore advantageous to further enhance the optical efficiency (and therefore the cost effectiveness) as well as to reduce the power losses (loss mainly as waste heat).
An important factor in the adoption of SSL is the light output efficiencies. While LED efficiency is nearing of fluorescent sources on the order of 60-100 lm/W, LED component costs are relatively high. As a result, SSL system designs focus on optimizing the light output of the SSL fixture in order to minimize the number of LEDs used in the system and the corresponding costs.
In conjunction with the energy efficiency and light control strongly desired of SSL systems, it is also desirable to be able to adjust light output levels and elicit light color control without loosing efficiency. Further, because LEDs are highly non-linear devices in their light intensity response to increased and decreased voltage, complex and potentially energy wasteful circuits are often used to “dim” and control light output of SSL fixtures.
Also, a primary need in SSL applications is high reliability over long periods of life. Solid state components like the CREE XRE LED emitter are sold with the assurance if properly applied and powered, they will produce 50,000 hours or longer useful life. It is very important the circuits used to power and control SSL light sources are themselves highly reliable and long lived.
Typically, power supplies for SSL lighting convert fixed ranges of line voltage (AC) to DC output. After attaining the necessary onset (turn on) voltage, SSL emitter light output (and sometimes color) is highly sensitive to voltage (Vf) applied across the emitter. For this reason, current limiting or controlling devices are often used with SSL circuits. Special power supplies, specifically designed for SSL lighting, are often equipped with current limiting as a built in feature. Dimming controls are optional to these power supplies, but usually come at a premium price and require an external control signal fed through a separate wire into the device (examples include Advance SignPro™ and Xitanium™ series LED Drivers). Externally controlled dimming power supplies require the electrician or fixture designer to add another device and circuit specifically for dimming control such as a low voltage rheostat or potentiometer.
In some cases to gain light output control, the main supply power is interrupted by a digital switch to effect pulse width modulation (PWM) control of average light output. Effectively PWM turns on and off the emitter circuit at a high frequency of repetition relying on visual persistence or persistence of the phosphors (found in many high intensity white LED emitters) to effect dimming. These systems are often associated with varying degrees of flicker or variation of light color. Further, PWM usually requires a separate timing control adding complexity, increasing size, reducing reliability, and consuming extra energy. PWM systems also (because of the switching of currents) create electromagnetic radiation possibly interfering with radio and other electronic devices.
SSL emitters come in a wide variety of colors. In white lighting applications there are several discrete options for color temperature and intensity of SSL emitters. It is often desirable to change net color output of an SSL fixture as either a function of intensity (similar to the color change an incandescent bulb goes through as it is increased in current or voltage) or to elicit special moods or lighting effects (warm light, moon light, sun light, etc.). Commonly, changing color is achieved by having multiple SSL emitters of different colors individually powered and dimmed (per the methods described above) or are simply turned on and off (via a remote switch or series of switches). Some lighting systems (e.g., Color Kinetics series of products) have an additional controller effecting gentle transitions from one color to another by combining dimming with controlled shutoff of individual emitters. However, these systems are generally complex, are costly to manufacture, have a number of points of failure and do not integrate well into existing wiring and dimming control systems used for other types of lighting.
Many existing fixture installations rely on standard AC dimming controls (such as rheostats, variable transformers and SCR or TRIAC chopping circuits) to supply amplitude or voltage waveform modified AC power to affect light output control. These are usually two wire systems (AC supply and return). It is highly desirable to have SSL lighting fixtures and systems compatible with these controls, without the need to rewire and replace existing circuitry.
Solid state lighting systems typically employ individual, strings, or arrays of emitters powered from either a direct current (DC) or alternating current (AC) source. Because SSL emitters are generally highly thermally sensitive and nonlinear as a function of current to voltage applied, and because it is highly desirable to maintain steady light output in most SSL applications (light output is proportional to current passing through the emitters), a variety of methods and circuits are often employed to limit or maintain current flow.
An SSL emitter is often characterized by a voltage level will be needed to start current flow through the device (initial turn on voltage) and by the forward voltage drop of the device at its desired operating current (Vf). Both of these levels are highly variable with temperature, from device to device, and from product to product. For instance, a popular LED manufactured by Nichia (NS6W083AT-E) manifests a Vf range of 3.2 to 4.4 volts at 300 mA and at 25° C. operating temperature. This range increases by ±10 percent as a function of operating temperature. Also, for this and many other devices like it, current will change at 20 to 60 times the rate of voltage change across the device.
In many SSL implementations, resistive devices (sometime referred to as ballast resistors) are placed in series with the emitters to help control current. Resistors are primarily linear in current response to voltage and help to hold a more steady current. Ballast resistors can also be chosen to be much more constant in current to voltage response as a function of temperature. Ballast resistors are simple and inexpensive. However, to achieve current control, resistors consume energy and in most implementations severely affect the energy efficiency of the circuit. Also, resistors do not fully control current variance as a function of voltage variance; they simply reduce the variance to a more linear result.
In many SSL implementation, a current limiting device or circuit (current regulator) is inserted in series with one or more emitters. There are two general types of current regulators, linear and switching.
A linear current regulator acts to change the resistance of the circuit in response to changing voltage by use of a feedback signal, such as detected voltage across a small resistor. A linear current regulator can control the current very accurately in response to voltage and temperature change. Linear regulators are more expensive and may require more components than using simple resistors to control current. And, because linear regulators behave mainly as variable resistors, they can be just as costly in their consumption of energy, particularly if the stacked Vf of the emitters is substantially lower than the voltage applied at the input of the regulator. Voltage drop across the regulator is maintained by converting electrical energy into waste heat. However, in general, linear regulators are simple and very reliable and robust.
Switching regulators also use feedback to control current, however they function quite differently from linear regulators. Inductors or capacitors are used in these regulators to store and recover energy. One or more transistors or switching devices are used to store or discharge energy in response to a feedback signal in order to maintain generally constant current. Switching regulators are never completely efficient, but are much better at conserving energy when the stacked Vf of the emitters is substantially lower (or higher) than the voltage applied at the input of the regulator. However, switching regulators suffer five significant drawbacks. Switching regulators generally involve more components and are almost always more expensive than linear regulators. Switching regulators are not more efficient than linear regulators when small voltage drops are involved. Switching regulators generally produce electromagnetic interference (EMI) and in some cases audible noise as the result of their switching frequency and components. Switch regulators generally require more circuit board area and have taller components than resistors or linear regulators. And, switching regulators are less reliable than other choices because of additional complexity and aging effects and environmental sensitivity of inductive and capacitive storage devices.
Ballast resistors are sometimes used in series with SSL emitters in combination with other current regulation devices to reduce voltage drop (and therefore power and heat dissipation) at the regulator, to reduce the non-linear load of the emitters on the regulator, and to compensate for large Vf differences between emitter loss.
In SSL applications, it is highly desirable to have a power and control circuit which have one or more of the following characteristics: monitor the voltage of the connected power supply or an independent control signal to control the number of emitters currently enabled in each string in accordance with the available power or voltage; work in conjunction with common current regulating devices and circuits to maintain high efficiency by balancing the number of emitters or ballast resistors on the available power or voltage; can be implemented in reference to either the high side (current in) or the low side (current out) of the power supply feeding each emitter string; enable a highly energy efficient system, by matching the number of enabled emitters to the available voltage or power, by minimizing voltage drop across non-light producing power components; enable energy savings because power supplies can be sized smaller since the system adapts to voltage degradation because of wiring, connector and time dependant power supply losses; can be configured to turn on and address either individual emitters or groups of emitters and can precisely control the sequence of turning on emitters as a function of input voltage or a control signal; can maintain light output as a near linear function of voltage; enable control the color mix of emitters turned on or, as light intensity is increased, discretely change the color of light; can be simple and reliable, utilizing high reliability discrete electronic components; respond with highly reproducible color and light output levels; can receive either DC or AC power without heating or performance problems; use AC power sourced through a conventional AC dimming control from line voltage (rheostat, variable transformer, TRIAC (TRIode for Alternating Current) chopping dimmer, etc.), a voltage reducing electronic or magnetic transformer; operate in changing voltage and brown out conditions without damage or shutdown; enable two or more discrete light output settings, each with their own brightness and color selection and provide more than one light output level, one of which is set to a level of light for emergency or safety lighting operating from a backup power source.